1. Field of the Invention
The present invention relates to a rectangular plate optical reflecting mirror in an optical scanner mounted on an image forming apparatus, such as a copier, a printer, or a facsimile. The optical scanner exposes a surface of an image bearing member with an optical beam (for example, laser beam) and scans the surface with the optical beam, the optical reflecting mirror reflecting the optical beam to guide the optical beam onto a surface to be scanned. Also, the present invention relates to the optical scanner and the image forming apparatus including the optical reflecting mirror.
2. Description of the Related Art
Typically, an optical scanner that is used in a copier or a printer exposes a surface of an image bearing member, for example, a photosensitive drum or a surface to be scanned, to light while scanning the surface, and forms a predetermined electrostatic latent image on the surface of the photosensitive drum. In the optical scanner, an optical beam, for example, a laser beam, emitted from a light source is deflected by an optical deflector in a main-scanning direction, and is sent to the surface to be scanned by an optical system member such as a optical reflecting mirror.
More specifically, typically, the optical scanner includes a light source such as a laser diode. The laser beam emitted from the light source is incident on a polygonal mirror serving as the optical deflector. The polygonal mirror reflects the laser beam using a reflection surface, and deflects the laser beam in the main-scanning direction. The laser beam that is deflected in the main-scanning direction is deflected at a uniform velocity by an fθ lens so as to be parallel to an axial direction of the photosensitive drum. The deflected laser beam is sent to the surface of the photosensitive drum via the optical reflecting mirror and an image is formed on the surface.
In recent years, image forming apparatuses capable of high-speed printing have markedly spread. The optical scanner has to operate to meet the performance of these image forming apparatus. Thus, the polygonal mirror has to rotate at higher speeds. Accordingly, vibrations are likely to occur at positions around the optical deflector in the optical scanner. Also, vibrations can result from the high-speed printing and occur at another position in the image forming apparatus and propagate to the optical scanner. Thus, the optical scanner mounted on the image forming apparatus has a greater likelihood of vibrating due to a plurality of vibration modes.
The vibrations occurring in the optical scanner or propagating to the optical scanner may reach the optical member, for example, the lens or the optical reflecting mirror. In particular, when the optical reflecting mirror vibrates, it is difficult to maintain the predetermined position and angle of the optical reflecting mirror. Accordingly, the optical axis of the optical beam may shift. As a result, the electrostatic latent image formed on the surface of the photosensitive drum may be adversely affected, thereby resulting in reduced image quality.
In light of this situation, to address the problems, various methods have been suggested to prevent the optical reflecting mirror from vibrating. Japanese Unexamined Patent Application Publication No. 10-282399 discloses an example of such methods. An optical reflecting mirror disclosed in the above publication includes a reinforcement member bonded to one or both side surfaces extending in a longitudinal direction.
Since the optical reflecting mirror disclosed in the publication includes the reinforcement member bonded to one or both surfaces extending in the longitudinal direction, this results in an increase in the weight and rigidity of the optical reflecting mirror. Thus, vibration of the optical reflecting mirror is decreased, and it reduces a defect from occurring as a result of a reduction in the flatness of the optical reflecting mirror. However, using a method wherein a characteristic frequency is reduced by increasing the weight of the optical reflecting mirror, such as in the above method, the vibration of the optical scanner with the plurality of vibration modes is not reduced. The optical reflecting mirror may resonate. Due to this, an image defect such as a lateral stripe may appear, and this may reduce the image quality.
Also, using a method such as bonding the reinforcement member to the optical reflecting mirror, may impact the flatness of the reflecting mirror due to the material characteristic of the reinforcement member. In this regard, the flatness of the optical reflecting mirror may be determined in view of the flatness of the reinforcement member, or the optical reflecting mirror may be deformed when the reinforcement member is deformed due to changes in temperature and humidity. Accordingly, an image defect such as curvature of field may appear, and this may reduce the image quality. Accordingly, image formation with high image quality may not be provided.
In light of the above, an advantage of the present invention is to provide a rectangular plate optical reflecting mirror which reflects an optical beam to guide the optical beam onto a surface to be scanned, the optical reflecting mirror having a rigidity that eliminates the necessity of using a reinforcement member, the reinforcement member being likely affected by material characteristic, capable of preventing resonance from occurring as a result of vibration with a plurality of vibration modes and capable of providing a high-quality image. Another advantage of the present invention is to provide an optical scanner and an image forming apparatus including the optical reflecting mirror.